Download PJ64, the latest version. Download plugins, (unless you're content with the ones given to you in the PJ64 package) and place them in the plugins folder, (already given to you if you downloaded the latest version). Then go to options, click on configure graphics plugin, and set it up the way your computer can handle it.
A good Graphics plugin is Rice's Video Plugin 6.1.0, but Jabo is just as good. Then click apply, and go to configure audio plugin, also set it up the way your computer can handle it. Another good Audio plugin is Azimer's HLE Audio v0.56 WIP 1, but again, Jabo is just as good.
Now, click ok/apply, and go to options and click on configure controller plugin. Then set up the keys to match your preferences on the keyboard, joypad ect. A good plugin is N-Rage's Direct-Input8 V2 1.80a, but again, Jabos is definately as good.
You don't need to worry about the RSP plugin. Now, download a game, and put it into a folder on the desktop, in the PJ64 folder, or somewhere, where you can make a small collection.
Click on File, then choose rom directory. Open up that folder, that has your game in it. Then press F5. Everytime you download a game into this folder, make sure you press F5, to refresh the rom list.
Now double click on the game that you want to play. If the sound doesn't work, configure the audio plugin to your liking. If your computer doesn't like the selected plugin due to incompatibility, then download another. The same with Video, if the picture garbles, or something screws with the graphics, then configure away. I hope this helps...